A turbocharger may be used to increase the power output of an internal combustion engine. The turbocharger does so by pressurizing the intake air, thereby increasing the mass of air provided to each of the engine's combustion chambers during the intake stroke. The increased air mass supports combustion of a correspondingly greater amount of fuel delivered to each combustion chamber, which provides increased power relative to a naturally aspirated engine of similar displacement. In a motor vehicle, a turbocharged engine may provide increased fuel economy by maintaining a higher power-to-weight ratio than a naturally aspirated engine of similar output and recovering internal energy from the exhaust to drive the turbocharger compressor. In addition, the use of a turbocharger allows a given power output to be achieved with a smaller or downsized engine. As such, the combination of engine downsizing, turbocharging, and delivery of fuel via direct injection has yielded considerable improvements in part-load fuel economy for gasoline-fueled engines while maintaining or exceeding the power output of conventional naturally-aspirated engines.
A turbocharger compressor is, however, susceptible to surge. Surge occurs when a pressure ratio in the turbocharger compressor (viz., the ratio of the outlet pressure to the inlet pressure) is too great relative to the flow of air through the turbocharger compressor. Turbocharger compressor surge (TCS) is a dynamic instability mode that can generate air-flow and self-excited pressure oscillations of great amplitude in the air mass flow. Surge can lead to undesired noise and noise, vibration, and harshness (NVH) issues. In addition, surge can constrain the torque capability of the engine, and may even affect the durability of the compressor hardware. For example, surge can induce undesirable stresses in the turbocharger and the intake, including excessive torsional loading on the turbocharger shaft. Consequently, continued or excessive TCS may decrease the longevity of the turbocharger and/or the engine to which it is coupled.
Various attempts have been made to enable early detection of surge, so that it can be addressed in a timely manner. One example approach to expedite surge detection is shown by Shu et al. in U.S. Pat. No. 8,516,815. Therein, transient surge, triggered by transient torque conditions such as tip-outs, etc., is detected by processing (e.g., low pass filtering) a manifold pressure estimated by a pressure sensor and/or mass air flow estimated by a manifold air flow sensor over a range of frequencies. The processed output is compared to a threshold to enable a faster and more accurate detection of surge, thereby improving surge mitigation.
However, the inventors herein have recognized potential issues with such an approach. As one example, the signal processing does not take into account the thermodynamics of the pressure waves. As such, the thermodynamic and chemical conditions of the engine may affect the pressure and airflow outputs of the sensor, thereby distorting the surge detection results. Specifically, parameters such as absolute pressure, temperature, humidity, and composition of the charge (including the amount of recycled exhaust gas or EGR) may affect the processed output resulting in incorrect surge detection (e.g., surge going undetected, or false positive surge detection). On the other hand, it may be difficult, and computationally intensive, to calibrate each engine to compensate for the varying thermodynamic conditions. As a result, a surge line on a compressor map may be calibrated more conservatively to satisfy safe and robust operation among a fleet of vehicles. Further, based on operating conditions, there may be engine actuators that correlate with the surge frequency. For example, actuators such as an intake throttle or an EGR valve may excite the pressure response in the same frequency band as that of surge, making it difficult to distinguish their effect on engine pressure from that of surge. Further still, the surge detection responsive to pressure estimation may change with change in driver demand due to non-minimum phase (NMP) behavior. The NMP behavior of the pressure can be falsely tagged as surge during selected transients, such as during a tip-in.
In one example, some of the issues described above may be at least partly addressed by a method for detecting surge in a boosted engine, comprising: combining one or more of manifold pressure and manifold flow with throttle inlet pressure into an aggregate intake pressure; and adjusting an engine operating parameter responsive to compressor surge, the surge determined based on the aggregate intake pressure, and further based on intake temperature. In this way, compressor surge be identified earlier and more reliably, improving surge mitigation.
As one example, an aggregate intake pressure may be computed by combining at least two pressure and/or flow measurements before and after the intake throttle for the purpose of surge detection. In particular, the aggregate pressure is then filtered with a band-pass frequency corresponding to the surge frequency range. Alternatively, filtered (e.g., band-pass filtered or low-pass filtered) values of intake manifold pressure (MAP) or intake manifold airflow (MAF) may be combined with throttle inlet pressure (TIP) into the aggregate intake pressure. The cut-off (or pass band) of the filter is adjusted based on engine operating conditions, such as temperature, to account for the variation in a surge frequency band where surge oscillations may be expected. A surge intensity is then calculated from the aggregate intake pressure using classical wave theory, taking into account the intake manifold temperature (at the time of MAP measurement) and/or boost temperature (at the time of TIP measurement). For example, an amplitude of peak pressure oscillations is determined. If the surge intensity is higher than a surge threshold, it is further determined if there were any actuator events that may have affected the TIP response in the same frequency band as that of surge. If yes, the effect of the actuators on the TIP response is separated from that of surge, and the intensity is reassessed relative to the threshold. A surge mitigating action, such as the temporary opening of a compressor recirculation valve, is then triggered based on the actuator-corrected surge intensity being higher than the surge threshold.
The technical effect of correlating the intensity of intake manifold pressure waves with one or more other available engine parameters is that more accurate detection of surge is enabled, and faster surge mitigation is possible. In particular, by comparing a filtered output of a throttle inlet pressure sensor to a baseline that is adjusted based on the thermodynamic and chemical conditions of the engine, the NMP effect on a throttle inlet pressure signal is reduced, allowing the onset of surge to be detected more accurately. Noise contributions from actuators that generate a TIP response in the surge frequency band can also be reduced. Further, a more aggressive surge line can be calibrated on a compressor map.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.